Silver compounds have historically been used as antibacterial agents. In ancient Greece and Rome, silver coins were used to preserve water. In the 19th century silver nitrate was used to treat a variety of ailments, from typhoid to post-partum infections. Today a variety of applications utilize silver-containing materials including Ag-coated medical devices, dressings for chronic wounds and burns, cosmetics, food preservation and water treatment.
More recently, silver (Ag) nanoparticles have been used in many applications due to their antimicrobial, antiviral and antifungal properties. For example, silver-containing compounds incorporated in or on the surfaces of a vast range of medical devices, such as vascular, urinary, and peritoneal catheters, endotracheal tubes, sutures, and fracture fixation devices. Additionally, silver nanoparticles can be utilized in diagnostic instruments/sensors. For example, as shown in FIG. 1A, a known plasmonic biosensor 100 can include silver nanoparticles 103 formed over a glass substrate 102.
A corresponding characteristic Plasmon extinction spectrum 101 is shown in FIG. 1B. As shown for biosensor 100′, the silver nanoparticles 103 can be biofunctionalized with a biomolecule 105. A higher refractive index of the biomolecule than a surrounding buffer solution can force a red-shift in the Ag spectrum 101′ as shown in FIG. 1B. When a ligand analyte 107, for example, an amino acid such as cysteine, a tripeptide such as glutathione and selective monoclonal antibodies, binds on the functionalized nanosilver surface 105, the spectrum is further redshifted by Δλ as shown at 101″ in FIG. 1B. In other words, a biosensor response, can be triggered via a shift in wavelength upon selective binding of ligands to biomolecules functionalized onto the silver nanoparticles surface. It would be beneficial to provide a method for delivering metal nanoparticles onto substrates that form components of such medical devices.
Silver nanoparticles are also used in non-medical materials as well such as plastics (e.g., kitchen appliances) and fabrics (e.g., yoga clothing manufacturer Lululemon's SILVERESCENT LUON®). While not limited to any particular theory, it is believed that antibacterial mechanism of silver is includes (i) inactivation of proteins, enzymes and DNA by silver (Ag) attachment to groups containing sulfur or phosphorus, (ii) cell membrane and mitochondria damage, (iii) inhibition of respiratory processes, and (iv) generation of reactive oxygen molecules and free radicals.
Meanwhile, nanoparticles of various forms, shapes and properties have been incorporated with toners, such as electrophotographic toners, in order to provide a carrying mechanism for delivery onto substrates. The methods of making such toners must balance the process of making the toner particles in order to preserve their desirable characteristics with additional processes for incorporating the nanoparticles with the toner particles to provide additional advantageous characteristics to the final application. For example, U.S. Pat. No. 8,137,879, which is hereby incorporated in its entirety herein, discloses a toner that includes single crystal magnetic nanoparticles for use in printing characters in Magnetic Ink Character Recognition (MICR) technology.
Among the major factors affecting print quality in any toner based printing system are the narrowness of the toner particle size distribution, the mean particle size, particle shape, particle surface morphology and toner charge distribution. The narrowness of the toner particle size distribution has an effect on the toner charge distribution and this has an effect on the variation in performance particle to particle in the development process. Hence, the narrower the toner particle size distribution, the more consistent the toner imaging performance will be in image development. Accordingly, there exist various methods for manufacturing toner according to the desired properties thereof. For example, in emulsion polymerization, monomers are diffused into a micelle where free radical polymerization proceeds with the resulting formation of polymer particles. In suspension polymerization, mechanical forces in mixing during polymerization dominate toner particle formation and hence particle size and distribution.
Some of the manufacturing methods are limited. For example, in emulsion polymerization toner manufacture, other necessary components of toner pigment, charge control agent, wax, etc. cannot be internalized into the polymer particles because such materials cannot diffuse into the micelle. However, if toner particle formation is attempted by direct combination of these components at the emulsion polymerization step, then they will reside on the polymer particle surface where they will affect the dispersion stability of the emulsion polymerization and cause erratic coagulation.
Accordingly, the process of preparing microspheres that enable control of well-defined particle characteristics such as size, size distribution and functionality are becoming increasingly important for a variety of applications. However, either the particle size range that is achievable or the types of materials that can be utilized in the process limit some of the current methods of microsphere preparation. The development of new methods for the preparation of microspheres that broaden the design space would therefore be an asset.
Emulsion aggregation (“EA”) processes provide for the preparation of micron-sized polymeric microspheres with narrow particle size distribution. With EA, it is possible to achieve a better Geometric Size Distrubition (GSD) than conventionally prepared toner which is significant in terms of imaging performance. The narrowness of particle size distribution, combined with the evenness of shape and homogeneity of emulsion/aggregation toners helps to create a narrow charge distribution. Additionally, small mean particle size toners tend to be more expensive to produce than larger ones when using conventional toner preparation methods. This cost progression tends to be geometric with the reduction in particle size in attrition grinding and conventional classification. However, in emulsion/aggregation manufacturing, there is essentially no relationship between mean particle size and cost. Thus, under the right conditions, there is the potential for an EA toner to provide better print quality and be more competitive in cost than a conventional toner.
In addition, the enablement of small mean particle size leads to the possibility of reduction in developed toner mass per unit area. This means that the amount of toner used per page is able to be decreased with consequent cost savings in total cost of ownership per page.
The shape of the toner also affects the toner flowability, charging and adhesion force. The combination of improvements in these attributes determines important performance factors such as transfer efficiency, developability and photoconductor surface cleanability. There are a few methods proposed for the determination and metrication of shape. One such method is to describe the “Shape Factor.” The shape factor of a toner particle is measured by comparing the square of the maximum length of a particle (ML) to the maximum projected area (A). The formula for shape factor is:Shape Factor (SF)=((ML)2/A)×(π/4)×100
Thus with highly rounded toners the shape factor is close to 100. With such toners it is possible to achieve very high transfer efficiency rates, in excess of 99%. The adhesion force between a toner and surfaces in the engine, such as photoconductor and intermediate transfer belt, is minimized by the uniform shape and surface of the toner. These properties not only lower adhesion force but also help to create uniform charging properties particle to particle. High levels of transfer efficiency mean that the consumption of toner and level of waste can be minimized. This type of performance translates into high yield and helps to lower the toner element in the cost of printing. However, residual toner after transfer with shape factors of 100 that remains on the photoconductor surface is more difficult to clean using the common blade cleaning technique. In practice the toner manufacturer is able to optimize the toner shape with emulsion/aggregation according to the application.
In broad terms, the EA process accomplishes the manufacturing of toners having desirable results by allowing for the controlled growth of microspheres from nanometer size constituents, such as polymer and pigments, through careful control of chemical and physical conditions to affect particle size, shape and size distribution. Additionally, a variety of resin types can be used in this process and other materials such as pigments can be incorporated into the particles. Thus EA provides advantages over mere emulsion polymerization toner manufacture and suspension polymerization. That is, in EA, the pigmentation and polymerization steps can be separated. Accordingly, there is no interference by the other toner ingredients with the polymerization process. Additionally, the ability to adjust and finely control the chemistry of the polymer and the other materials in the toner particle formation step in emulsion aggregation permits the particle size and particle size distribution to be more controllable.
The Emulsion/Aggregation process begins with the preparation of nanometer sized polymer particles stabilized in water using various techniques. These particles can be on the order of 10 to 300 nm in size. A variety of resin types can be used, including styrene-based materials, acrylates, polyesters, etc. A second step involves the growth of the nanometer-sized particles by mixing in deionized water in the presence of an aggregating agent. It is at this stage that other ingredients can be incorporated into the particle by adding them as water based dispersions. All of the components are homogenized to ensure effective mixing and continuous mixing is utilized throughout the growth process. Once the desired particle size is reached, the growth process can be terminated. Depending on the resin type utilized, the particles generated at this stage are either already spherical or require further treatment to coalesce into spheres. Once the microspheres are formed they can be isolated from the water and washed to remove the various ions and surfactants used in the process.
The EA process enables control of particle size and size distribution. As particles grow, the particles size increases with time. It is also during this phase that the particle size distribution narrows. A narrow particle size distribution is achieved using the EA process and a typical size distribution curve. The geometric standard deviation based on volume is less than 1.25. The ability to obtain narrow particles size distributions is driven by the growth kinetics and balance between the forces binding the particles together and the shear forces that erode aggregated particles. The growth process proceeds from individual particles, to a gel network, to individual aggregates that continue to grow with time.
The particles produced in this process can vary in shape depending on process conditions. For the styrene-based toner particle case, the conditions can be adjusted such that a non-spherical or completely spherical particle is obtained. The EA process is particularly suited to the incorporation of nanometer-sized pigments. This has been demonstrated in the application to toner materials for electrophotographic applications. Dispersions of pigments in water are mixed with the emulsified resin and the process is carried out in the same manner. Cross sections of particles that contain pigment show that the pigment is fairly evenly distributed throughout the particle. It is possible to add additional latex to cover surface pigment.
An EA process is summarized in a flowchart illustrated in FIG. 2. As a preliminary step in forming primary resin particles, a monomer mixture is formed 201 by blending a monomer such as that of styrene with an acrylic ester and acrylic acid 202 in a low speed mixer for an appropriate amount of time to ensure homogeneity. An aqueous medium phase containing other polymerization ingredients is also prepared 202 in a mixer and can contain, for example, hot deionized water with an anionic surfactant, initiator and chain transfer agent. In the manufacture of a latex via polymerization 203, an emulsion is formed by mixing the two phases from 201 and 202 at elevated temperature in a low intensity mixer for a predetermined number of hours. In this process “primary particles” are formed. These can be in the form of a latex of non-pigmented emulsion polymerized particles between 0.1-0.3 microns.
The process continues with preparation of a pigment dispersion and other components formed in an aqueous medium at 204. That is, a pigment in deionized water can be prepared with small amounts of a dispersing aid and an anionic surfactant. In addition to or alternatively, a wax component can be dispersed therein by high shear mixing and applied heating of this material in deionized water with small amounts of dispersing aid and anionic surfactant.
The mixture of primary particles in their aqueous medium can then be transferred to a high-speed jacketed mixer and the dispersions colorant and wax and a charge control agent (CCA) can be added at 205. A metal compound coagulant can also be added to the mixture. The mixture can then be cooled and then dispersed for a further predetermined amount of time. Secondary particles are formed by the agglomeration of the solids in the aqueous medium containing primary particles, the pigment, wax and CCA. The particle size at this stage of production can be about 1.0-4.0 microns, typically about 2.5 microns.
During a coalescence step 206, enlargement of the toner particle aggregation formed in step 205 can be accomplished via, for example, further stirring at elevated temperature for a predetermined number of hours. The homogenized mixture can be heated with continuous mixing in the reactor and with gradual ramping up of the temperature until it reaches about 90° C. where it is then further mixed and held at this temperature for about 4 hours. The secondary particles continue to grow under these conditions. The process of toner particle formation is complete at this point. However, if the toner is to be encapsulated when, for example, the toner particles are slightly smaller than the desired finished size, a shell-forming latex solution formed in 205′ can optionally (as indicated by the dashed line) be added to the aggregation formed in 205. When the particles reach the desired size, the pH of this aqueous mixture is adjusted to stop the process.
Shape adjustment (not shown) can optionally be conducted at this stage by adjustment of the temperature and other conditions. Increasing the temperature to above the Tg, for example, controls the viscosity of the heated polymer and allows interfacial interactions and surface tension to be used to change the particle shape. The particle shape may be changed from irregular to spherical by altering the conditions and stopping the process when the desired shape is achieved.
The mix can then be filtered, washed, and dried at step 207 to yield a “pre-toner”. Methods of washing and drying a re-slurry are known and applicable in this conventional EA method. Generally, these are predominantly batch processes with a re-slurry step after each wash and used of copious amounts of deionized water. After drying, surface additives such as fumed silicas are blended in 208 to provide additional flow and charge characteristics to the toner.
Through an EA processes such as that which is disclosed in U.S. Patent Application Publication No. 2011/0200927 (which is hereby incorporated in its entirety herein), an electrophotographic toner comprising spherical metal nanoparticles has been provided. In other EA processes such as those disclosed in U.S. Patent Application Publications Nos. 2012/0202148 and 2010/0086867 (which are hereby incorporated in their entirety herein), toner particles having a core-shell structure have been provided. However, such methods are limited because they do not provide for carrier toner particles that allow for delivery of metal nanoparticles onto a substrate such that the metal nanoparticles are localized on a surface when delivered onto a substrate.
What is needed in the art, is a toner particle that can provide antimicrobial, antiviral and antifungal properties, a method of making the same, and print articles that include such toner particles.